Method for making isobutyric acid

ABSTRACT

What is disclosed is a continuous one-step method for the preparation of isobutyric acid or a lower alkyl ester thereof by the Koch synthesis, which method comprises reacting propylene, carbon monoxide, and water or a lower alcohol, in the gaseous and liquid phases, in the presence of hydrogen fluoride as a Koch catalyst, at a temperature between 80° C. and 160° C., at a dwell time of the liquid phase of less than 20 minutes, and with a high degree of backmixing, while maintaining the content of propylene in the reaction mixture at less than one percent by weight of the liquid phase, wherein all or part of the propylene, carbon monoxide, and water or lower alcohol can be replaced by binary addition compounds formed pairwise between these materials. In the alternative, isopropylformiate can be continuously rearranged in the presence of hydrogen fluoride to produce isobutyric acid.

This application is a continuation of Ser. No. 290,601 filed Aug. 6,1981 and now abandoned, which in turn is a divisional of Ser. No.215,523 filed Dec. 11, 1980 and now abandoned.

The present invention relates to an improved method for makingisobutyric acid or its lower alkyl esters. These compounds can beconverted by dehydrogenation into methacrylic acid or its lower alkylesters, which latter materials are in turn starting materials forvaluable synthetic resins.

The preparation of isobutyric acid by the Koch synthesis, starting frompropylene, is known in the prior art. When water is used, isobutyricacid is obtained; when lower alkanols are used, particularly methanol,the corresponding lower alkyl esters are formed. One can also start withisopropyl alcohol and in this way obtains isobutyric acid without theuse of water. As a Koch catalyst for these reactions, hydrogen fluoride,inter alia, is used.

The preparation of isobutyric acid or its esters by the Koch synthesishas until now not been realized on a technical scale. Although thereaction has been studied thoroughly and numerous method variants andexperimental conditions have been tested, the high selectivity and yieldcoupled with a tolerable dwell time or space-time yield necessary for atechnical process have not been achieved.

According to other prior art, propylene is converted to isobutyric acidin a two-step process. In the first method step, propylene and carbonmonoxide are reacted in anhydrous hydrogen fluoride. The intermediateproduct is reacted in the second reaction step with an alcohol to forman ester. Although the first reaction step is concluded in 15 to 20minutes, the second reaction step in general requires a longer reactiontime. The total time for the method as a rule exceeds one hour. At attotal reaction time of 30 minutes, only about one-half of the propyleneintroduced is converted into the isobutyric acid ester.

Considerably higher yields of carboxylic acids are obtained according toa one step method known in the art in which an approximately equimolarmixture of an olefin and water with carbon monoxide is reacted in alarge excess of hydrogen fluoride serving as the catalyst. In order toobtain yields on the order of magnitude of 90 percent, reaction times oflonger than 30 minutes, mostly longer than 60 minutes, are necessary attemperatures from 23° C. to 38° C.

Still other prior art teaches a method proceeding in the presence of asimilarly large amount of hydrogen fluoride which contains from 5 to 30percent of water or, if the preparation of the corresponding carboxylicacid ester is intended, of 2 to 15 mol percent of alcohol, at atemperature between -12° C. and 93° C. In this method, the temperatureshould be higher the higher the water content in the reaction mixture.Although the reaction time is reported as being in the general latitudeof one minute to one hour, the practical performance of the method as arule requires more than one hour.

Y. Takezaki et al. (Bull, Jap. Petrol. Inst. 8 3-38 (1966) havesystematically investigated the Koch synthesis of isobutyric acid frompropylene, water, and carbon monoxide in hydrogen fluoride and havedetermined an optimum for high yield and high reaction velocity.According to this optimum, 15 mols of hydrogen fluoride, which are mixedwith 20 percent by weight of water, are employed per mol of propyleneand the reaction is carried out at 94° C. under a total pressure of 190atmospheres or more. Under these conditions, a yield of isobutyric acidwhich is 70 percent of theory is obtained in 20 minutes. At a higher orlower water content, the yield decreases, as it does also using a higheror lower reaction temperature. With increasing temperature, theformation of undesired oligomerization products of propylene increases.At pressures of 50 to 100 atmospheres, yields of only about 50 percentare obtained.

According to still other prior art, olefins having four or more carbonatoms are reacted with carbon monoxide and excess water at 20° C.-90° C.in the presence of hydrogen fluoride. Propylene is excluded as astarting material. In this method, high yields and selectivities for thecorresponding carboxylic acids are obtained, particularly in acontinuous mode of operation, if a saturated hydrocarbon is used as adiluent for the olefin. The necessary dwell time is 40 to 60 minutes.

From other prior art it is known to isomerize isopropylformiate underhigh pressure and at high temperature in N-methyl pyrrolidone withcobalt bromide as a catalyst. In contrast to the Koch synthesis, amixture of iso-butyric and n-butyric acids is obtained according to thismethod, which mixture is difficult to separate distillatively. The totalyield of both acids is 76 percent.

The long reaction times in the known discontinuous processes and theequally long dwell times in the aforementioned continuous processsuggest that a continuous Koch synthesis of isobutyric acid frompropylene, carbon monoxide, and water on a technical scale shows littlepromise, since in this case short dwell times and high yields andselectivities are indispensable. The finding reported by Takezaki et al.(loc. cit.) that the oligomerization of propylene increases considerablywith increasing temperature counterindicates increasing the reactionvelocity by an increase in the temperature above 94° C.

Indeed, the formation of oligomers proves to be the problem mostdifficult to overcome in the Koch synthesis with propylene. The factthat certain of the prior art discussed above is limited to the use ofolefins with more than three carbon atoms, and that other art mentionedearlier may claim a reaction of propylene, but does not show such areaction in a working example, is explained by this difficulty. Evenhigher olefins gives considerable, and often even predominant, amountsof oligomerization products, as is evident from the working examplesfound in the last-mentioned prior art patent. Even losses in yields offrom 8 to 10 percent in the form of worthless oligomerization products,as are encountered in all known methods for carrying out the Kochsynthesis with propylene, are unacceptable for the preparation of abasic material for the manufacture of synthetic resins.

The present invention has as its object the performance of thissynthesis under conditions which permit a large-scale method for thepreparation of isobutyric acid or its esters from propylene, carbonmonoxide, and water or an alcohol. This includes a requirement for acontinuous mode of operation, high yield and selectivity and therewithan extensive diminution of the oligomer problem, as well as a demand fora higher space-time yield, i.e. a short dwell time.

The recognition that high temperatures of at least 80° C. andparticularly over 100° C.--which per se promote the formation ofoligomers--were unavoidable in the interest of achieving a high reactionvelocity preceded a solution to the problem. It was found that evenunder these conditions, oligomer formation can be significantly limitedif the propylene concentration in the reaction mixture is continuouslyheld below a low level. The realization of this principle led to themethod described and claimed therein. A continuous mode of operationsets the condition for the introduction of propylene according to thesize of the batch in process. A high backmixing of the reaction mixtureis necessary in order to prevent any local increase in concentration atthe point at which the propylene is introduced.

The propylene concentration is adjustable, at whatever reaction velocityprevails, by the speed of introducing the starting materials and, inthis way, practically by the dwell time. The dwell time must in eachcase be sufficient for the almost complete reaction of the propylenewhich is introduced. By the term "propylene concentration" is to beunderstood the percentage content by weight of propylene in the totalweight of the reaction mixture consisting of the liquid and gaseousphases.

It was not foreseeable that this problem of oligomer formation could besolved in a satisfactory manner by the claimed continuous method ofoperation at propylene concentrations below one percent, and preferablybelow 0.7 percent.

It has proved advantageous to maintain the content of water, or of alower alcohol introduced for ester formation, in the reaction mixture atless than 5 mol percent, calculated on the hydrogen fluoride, andparticularly below 2 mol percent. This is controllable by means of theamount of water or the amount of alcohol which is introduced. In thisway, oligomer formation is suppressed further and the reaction velocityis increased at the same time, which in turn permits shorter dwelltimes. The mol ratio of propylene to water or alcohol should preferablybe about equivalent, but can deviate from a 1:1 ratio to values between1:0.8 and 1:2, in order to maintain the water or alcohol content in thereaction mixture at the desired level. The ratio of propylene to wateror lower alcohol is advantageously 1:(0.9-1.1), particularly1:(0.95-1.0). Hydrogen fluoride is preferably added in an amount from 5to 15 mols, particularly from 7 to 12 mols, per mol of propylene or ofisopropyl alcohol introduced.

Under optimum conditions, the method according to the invention permitsthe attainment of a total yield of isobutyric acid compounds of morethan 90 percent of theory, of which more than 80% of theory is in theform of free isobutyric acid, at dwell times of only a few minutes andthereby opens for the first time the possibility of a technical scalesynthesis of isobutyric acid.

When the direct formation of an ester of isobutyric acid is sought, thenthe corresponding alcohol is introduced instead of water. Above all,lower primary alkanols having one to four carbon atoms are employed.Methanol is the preferred alkanol. For the conversion of isobutyric acidcompounds into methacrylic compounds, free isobutyric acid offersadvantages over its esters. Hence, the preparation of the free acid,rather than the preparation of isobutyric acid esters, is the preferredembodiment of the invention.

The preferred starting materials for the method of the invention arepropylene, carbon monoxide, and water or a lower alcohol. To the extentthat binary addition compounds are formed from pairs of the materialspresent in the reaction mixture, these addition compounds can be usedinstead of or in addition to the components from which they are formed.Propylene can form isopropyl alcohol with one mol of water and can formdiisopropyl ether with 1/2 mol of water. Isopropyl fluoride is easilyformed with hydrogen fluoride. Carbon monoxide can be reacted with waterto form formic acid, with a lower alcohol to form the correspondingformic acid ester, and with hydrogen fluoride to form formyl fluoride. Alower alcohol and propylene can give rise to the corresponding isopropylalkyl ether. These binary addition products react according to themethod of the present invention as easily as the components from whichthey are formed to produce isobutyric acid or its ester. Since they mustbe prepared in a special method step, their use, instead of or inaddition to the components from which they are formed, is recommendedonly in exceptional cases. Among the aforementioned addition products,isopropyl alcohol takes a preferred position because of the ease withwhich it is formed. For example, it is obtained from a mixture ofpropylene and water on an acid ion exchanger. There are also ternaryaddition products of three component materials, namelyisopropylformiate, which is comprised of propylene, carbon monoxide, andwater. Isopropylformiate takes a special position as a starting materialfor the method of the present invention because its use in the method ofthe invention is not strictly governed by the narrow reaction conditionswhich pertain to mixtures of the components which form it or to thebinary addition compounds.

The reaction of isopropylformiate to form isobutyric acid is a pureisomerization, so that no further starting materials are necessary. Inany event, it is preferred to carry out the reaction in the presence ofcarbon monoxide, which is not used in the reaction and which can becompletely recovered from the reaction mixture. The reaction temperatureis in general between 20° C. and 150° C., preferably between 60° C. and120° C. The dwell time in the reaction vessel can be between one minuteand 200 minutes. Hydrogen fluoride is introduced in an amount of from 5to 20 equivalents of acid per mol of isopropylformiate.

In the method of the present invention, the oligomerization reactionswhich occur extensively in other cases in which the Koch synthesis isused are largely absent, even in the method embodiment usingisopropylformiate, so that a selectivity for isobutyric acid which nears100 percent is attainable. Further, a practically quantitative reactionis attained.

Since the addition products in the reaction mixture in general decomposerapidly into their component parts, the limiting value specified hereinfor the propylene concentration, and, possibly, also for the water oralcohol concentration, must be observed in a corresponding fashion whendetermining the rate of addition or the dwell time.

The total pressure in the reactor is comprised of the partial pressuresof propylene, carbon monoxide, hydrogen fluoride, and of the vaporpressure of the organic substances present in the liquid phase. Thetotal pressure under the reaction conditions is preferably in the regionfrom 50 to 150 atmospheres, particularly from 80 to 140 atmospheres. TheCO partial pressure is at least 35 atmospheres, and preferably from 60to 130 atmospheres.

In the preferred embodiment of the invention, which employs a mixture ofpropylene, carbon monoxide, and water or alcohol, or theiraforementioned binary addition products, the materials are reactedcontinuously with a high degree of backmixing at a dwell time of theliquid phase of less than 20 minutes and at a temperature between 80° C.and 160° C., and the propylene content is maintained at less than onepercent of the weight of the liquid phase.

The method can be carried out in a broad temperature range from 80° C.to 160° C., wherein the reaction velocity increases with temperature.The preferred temperature region extends from 100° C. to 140° C.

With dwell times of from 2 to 15 minutes, total yields over 90 percentof theory and selectivity for isobutyric acid above 93 percent areobtained at temperatures of 100° C.-120° C.

The method of the invention can be carried out in conventional pressurereactors with high backmixing, which reactors must be resistant tohydrogen fluoride under reaction conditions. As materials for thereaction, nickel or nickel alloys, such as "Monel", "Inconel", or"Hastelloy" are suitable. The size required for the pressure reactor isdetermined by the amount of material to be passed therethrough hourlyand by the space-time yield. Since the cost of pressure reactors,particularly those made of special alloys, increases sharply with thesize, the short dwell time permitted by the invention and the highspace-time yields associated therewith advantageously affect plantcosts. Contrasted with a dwell time of an hour, as is necessary for manyknown methods, the dwell times which can be attained according to theinvention of, for example, 5 to 10 minutes, permit a decrease in reactorsize to between a sixth and a twelfth of the size otherwise needed. Thevolumes of safety vessels and relief vessels are decreased in a similarmeasure.

In addition to simple stirred autoclaves, gas-liquid reactors having adispersed gas phase come under consideration. Bubble columns or jetreactors, which facilitate an intimate contact of the liquid phase withthe gas phase and a high circulation rate of the liquid reactionmixture, are of this type. The gaseous components of the reactionmixture, particularly propylene and carbon monoxide, are preferablymixed before entry into the reactor and are introduced as a gas mixture.Gas can be removed from the upper portion of the reactor and can againbe mixed together with fresh gas for recycling.

The liquid phase of the reactor is continuously drawn off by way of alet-down valve and is worked up in the usual way. Preferably, the crudeproduct is treated distillatively. In such a treatment, the low-boilingcomponents, above all hydrogen fluoride and any unreacted propylene, aredistilled off in a first distillation step at normal pressure or at aslight overpressure of, for example, four atmospheres, compressed, andreintroduced into the reactor. If a short distillation column isemployed for this purpose, the hydrogen fluoride can be substantiallycompletely separated from the organic reaction products.

All low-boiling by-products are volatilized together with the hydrogenfluoride, for example isopropyl alcohol, isopropyl fluoride, isobutyricacid fluoride, isobutyric acid isopropyl ester, and diisopropyl ether.They are returned to the reactor with the hydrogen fluoride andparticipate in further conversion to isobutyric acid. In part, they arereadily decomposed with water. Thus, it is advantageous to add somewater to the crude product during distillation and thus to promote thehydrolysis of isobutyric acid fluoride or of isobutyric acid isopropylester to isobutyric acid. The coproducts of the hydrolysis, namelyhydrogen fluoride and isopropyl alcohol, are returned to the reactor.

The liquid portion of the crude product which remains after the firstdistillation step principally comprises isobutyric acid and smallamounts of high-boiling oligomers of propylene and the like. In a seconddistillation step, preferably in a further short column, the isobutyricacid is evaporated and either condensed in known fashionor--preferably--led directly in vapor form to a reactor for oxidativedehydrogenation with formation of methacrylic acid. Since this reaction,or the plant installation used therefor, can be disturbed by hydrogenfluoride, the vaporized isobutyric acid is advantageously passed over asuitable absorption column. For a complete removal of hydrogen fluoride,a bauxite filling is suitable, for example. The high-boiling ornon-volatile distillation residues are discarded or burned.

A better understanding of the present invention and of its manyadvantages will be had by referring to the following specific Example,given by way of illustration.

EXAMPLE 1 Continuous reaction of propylene, carbon monoxide, and water

The reaction is carried out in a stirred autoclave made of a nickelalloy ("Hastalloy C4") having a free reactor volume of 60 ml andequipped with a gas-treatment stirrer. The stirring velocity is about1,000 rpm. The autoclave is equipped with a gas inlet passing throughthe stirrer as well as further inlets and outlets and can be warmed withan electric heater.

Carbon monoxide is introduced in part through the gas-treatment stirrer.The remaining components are pumped in through separate conduits.

Propylene, water, technical grade hydrogen fluoride, and carbon monoxideare reacted at reactor temperatures of 100° C. and 120° C. and anoperating pressure of 120 atmospheres at a mol ratio of 1:0.98:10:1.5.The dwell time is varied between 4 and 15 minutes by alteration of thethroughput amount. In a stationary state, the following yields andselectivities are attained.

    __________________________________________________________________________    Reaction temperature = 100° C.                                                             Select-                                                                           Reaction temperature = 120° C.                 Dwell                                                                             Propylene                                                                           Yield     ivity                                                                             Propylene                                                                           Yield    Selectiv-                              time                                                                              conc. IBS*                                                                              Oligomers                                                                           of IBS                                                                            conc. IBS*                                                                              Oligom.                                                                            ity of IBS                             (min.)                                                                            (W. %)**                                                                            (%) (%)   (%) (W. %)**                                                                            (%) (%)  (%)                                    __________________________________________________________________________    4   0.31  81  5.5   86  0.12  91  3.5  95                                     7.5 0.12  82  3.8   92  0.07  93  1.5  96                                     15  0.09  94  1.9   96  0.04  94  2.0  96                                     __________________________________________________________________________     *IBS = isobutyric acid and derivatives which can be hydrolyzed to form        isobutyric acid, particularly the isopropyl ester.                            **propylene concentration in the reaction mixture in percent by weight of     the total reaction mixture.                                              

At a dwell time of 3 minutes, the propylene:water ratio was variedbetween 1:0.96 and 1:0.99. The following results were obtained at 120°C.

    ______________________________________                                        Propylene/water                                                                              Yield of IBS                                                                             Selectivity                                         (mol)          (%)        (%)                                                 ______________________________________                                        1:0.96         85         91                                                  1:0.97         89         93                                                  1:0.98         92         95                                                  1:0.99         89         93                                                  ______________________________________                                    

EXAMPLE 2 Continuous reaction of isopropylformiate and carbon monoxide

A reaction mixture of isopropylformiate, technical grade hydrogenfluoride, and carbon monoxide in a mol ratio of 1:10:0.5 were reactedcontinuously in the apparatus described in Example 1 at 70° C. and at120° C.

    ______________________________________                                        Results:                                                                      ______________________________________                                        Reaction temperature                                                                         70° C.                                                                              120° C.                                    Dwell time     30 min.      5 min.                                            Pressure in the reactor                                                                      100 atmospheres                                                                            110 atmospheres                                   Yield of isobutyric acid                                                                     97%          98%                                               Selectivity    greater than 98%                                                                           greater than 98%                                  ______________________________________                                    

EXAMPLE 3

Following the procedure of Example 1 propanol-2, hydrogen fluoride, andcarbon monoxide in a mole ratio of 1:10:1.5 are reacted at 120° C. andan operating pressure of 120 atmospheres, the dwell time being 15minutes. Under stady state conditions IBS (as explained in Example 1) isobtained in a yield of 94%. The selectivity is 97%, calculated on thepropanol-2 feed. Oligomers are formed in a yield of less than 2 percentof the propanol-2 feed.

EXAMPLE 4

Following the procedure of Example 1 propylene, ethanol, hydrogenfluoride, and carbon monoxide in a mole ratio of 1:1.2:10:1.5 arereacted at 110° C. and an operating pressure of 130 atmospheres within adwell time of 15 minutes. Under stady state conditions isobutyric acidethyl ester is produced in a 89 percent yield and a selectivity of 92percent, calculated on added propylene. Oligomers are formed in a yieldof about 2.5 percent of the propylene feed.

What is claimed is:
 1. A continuous one-step method for the preparationof isobutyric acid which comprises continuously rearranging isopropylformiate in the presence of a catalyst consisting essentially of 5 to 20equivalents of hydrogen fluoride per mol of isopropyl formiate at atemperature between 20° C. and 150° C. at a dwell time of said isopropylformiate between one and 200 minutes.
 2. A method as in claim 1 whereincarbon monoxide is present.